maybe someone knows: You have a 70 minute MP3. You start the car again and start the MP3 player from the start. Now take the time to rewind to the time before where you left off. This can be annoying after a while!

Is it somehow possible, modifying the music player that we have a music bar to fast forward or rewind to do?

Functional hacks and mods can only be done with the navi portion of the unit. So the answer is no.